Your Android device is not recognised (unlikely but possible)See (4) below. Usually this is for low volume devices / devices that are newly launched.Request it be added at this link -

Your version of libmtp is not recent enough.If running Debian 6 then upgrade to Debian 7 (wheezy)If you are running Ubuntu 12.04 then find a backport / ppaOR for the adventurous compile libmtp yourself.

And lastly (5) "You have libmtp installed but not libmtp-runtime"

libmtp-runtime is essential, but is only listed as a 'Recommends' when installing Gmtp

Solution: apt-get install libmtp-runtime

How would I know that libmtp-runtime is missing?

Your package manager can tell you certainly, but one symptom is an entry in the system log that says:

failed to execute '/lib/udev/mtp-probe'

For those interested in dependency details, a screenshot next (see last line in screenshot)

Screenshot shows that libmtp-runtime is only a 'Recommends' of libmtp

Notes and Further Reading:

If you use Ubuntu then you should prefix the apt-get commands shown above with 'sudo'